parallel version of hnsw

[jianshu93]

Hello rust-cv hnsw team,

It seems the build and search process is not parallelized. Despite the fact that the reference c++ complementation in the original paper is also not parallelized, they do provide task level parallelism using python (which is more easy than C++). In Rust, rayon or cross-beam can be use to parallelize any the build and search of hnsw structure. Any plans in the future to also add parallelized version? As the number of database size grows even larger (like 10^6 or above), fast search using all processors is necessary.

Thanks,

Jianshu